On a traditional solid wall, the question is not which insulant performs best but which one lets the wall dry. The tabulated figure that settles it is the water vapour resistance factor µ. It is 1 for mineral wool and 150 for extruded polystyrene, a ratio of 150 between two products the same quotation often sets side by side. At 120 mm thick, that is 12 cm of equivalent air on one hand and 18 metres on the other.
What really sets a pre-1919 wall apart
A traditional wall has no damp-proof course and no waterproof barrier within its thickness. It is built straight off the ground, usually in lime mortar, and it takes water by three routes: driving rain on the face, rising damp at the base, and vapour generated inside the dwelling. It was never meant to stay dry. It was meant to dry out.
That drying happens through both faces. Where the external render has been redone in cement, which covers a good share of the stock retrofitted in the 1970s and 1980s, the outer face no longer dries. The internal face becomes the only way out, and it is an effective one because it is heated.
That is exactly the face internal insulation closes off. The insulant cools the masonry, which lowers its saturation vapour pressure, and the lining puts a diffusion resistance in the way. Water keeps arriving by the other two routes and has nowhere left to go.
The table that settles it: µ and Sd of insulants
The water vapour resistance factor µ is dimensionless: it says how many times the material slows vapour compared with a layer of still air of the same thickness. The figure that matters on site is Sd, the equivalent air layer thickness in metres, equal to µ times the thickness installed.
| Insulant | µ dry | µ wet | Sd at 120 mm | As a lining on traditional masonry |
|---|---|---|---|---|
| Glass or rock mineral wool | 1 | 1 | 0.12 m | Open, but tolerates no liquid water |
| Expanded polystyrene (EPS) | 60 | 60 | 7.2 m | Rule out bonded to a traditional wall |
| Extruded polystyrene (XPS) | 150 | 150 | 18 m | Equivalent to a vapour barrier, avoid |
| Polyurethane, vapour-permeable facings | 60 | 60 | 7.2 m | Rule out, unless a ventilated cavity |
| Cellulose, wood fibre, hemp | no default value | Read the value on the product certificate |
The first four rows are default values. The fifth is not one: for plant-fibre insulants the fascicle tabulates nothing and refers to the annexes of the thermal characteristics orders. In other words a bio-based product has no generic regulatory value, and its permeability is read product by product on its certificate. That is a paperwork constraint, not a defect of the material.
The order of magnitude to hold on to: a vapour check sits between 2 and 18 m of Sd, a vapour barrier above 18 m. A 120 mm XPS board bonded to the wall therefore reaches the Sd of a vapour barrier on its own, and it does so on the wrong side of the insulation, against cold masonry.
What the wall you are lining is already worth
The other figure the survey has to produce is what the wall already contributes. The tabulated conductivities are equivalent values, joints included.
| Masonry | λ in W/(m·K) | µ dry | µ wet | R of a 500 mm wall |
|---|---|---|---|---|
| Very soft limestone | 0.85 | 30 | 20 | 0.59 m²·K/W |
| Soft limestone, grades 2 and 3 | 1.10 | 40 | 25 | 0.45 m²·K/W |
| Firm or semi-firm limestone | 1.40 | 50 | 40 | 0.36 m²·K/W |
| Hard limestone | 1.70 | 200 | 150 | 0.29 m²·K/W |
| Granite | 2.80 | 10,000 | 10,000 | 0.18 m²·K/W |
| Solid fired clay brick, 1,800 kg/m³ | 0.69 | 16 | 10 | 0.72 m²·K/W |
| Rammed earth, cob, compressed earth blocks | 1.10 | not tabulated | 0.45 m²·K/W |
Two lessons. First, the gap between soft limestone and granite is a factor of three on λ: writing "a 500 mm stone wall" on a quotation without saying which stone says nothing at all. Second, hard limestone and granite are themselves very closed to vapour, which moves the problem: on those walls the risk is no longer the drying of the masonry but accumulation at the interface between the insulation and the cold wall.
What you carry afterwards
A damp failure that follows internal insulation lands on the installer as soon as the dwelling is no longer fit to live in, and the presumption runs against the contractor. The French agency Anses established in its 2016 collective appraisal that the respiratory effects of exposure to mould are proven, and that 14 to 20 % of dwellings show visible mould. A home made uninhabitable by fungal growth after work therefore needs little argument to be qualified.
Three records change the outcome of a dispute, and all three are made before installation:
- the dated opening-up survey, establishing the true construction and thickness;
- the moisture reading of the masonry at the base and at mid-height before work;
- the written caveat on the quotation covering the external render and the existing ventilation.
The order of work, and why it is not negotiable
On a traditional building the order is not a matter of method preference, it is what decides whether the failure happens at all.
Water first: rainwater discharge, the base of the wall, and the external render if it is cement. Ventilation next, sized and measured, because a wall that no longer dries outwards needs the internal air to carry the vapour away. Insulation only then, followed by the heat source recalculated on the post-works heat loss.
A lining fitted before the ventilation is compliant turns a damp home into a mouldy one. On that point, dealing with rising damp at the base of the wall and choosing a breathable system are handled together, never one after the other.
Masonry types, case by case
Each family of traditional wall has its own failure mode, and the precautions do not transfer from one to another. Stone walls call for precautions specific to the stone and the bedding mortar. Wattle and daub on a timber frame raises the question of contact between insulant and timber, where the technique chosen decides whether the frame survives. Rammed earth loses its cohesion if it is wetted for any length of time, which rules out any solution that dampens it even temporarily.
In conservation areas external insulation is often refused, which leaves internal lining as the default. That is no reason to apply the same solution as on a 1970s house: the constraints on a listed building bear on appearance, not on the physics of the wall, which remains that of a breathable element.
Heat loss calculations and the build-up of the element are worked from what the survey actually records, and Argile reconstructs the construction date, the massing and the planning constraints from the address alone before the visit even happens.
What to take away
- A traditional wall dries through its internal face, and that is the route the lining closes.
- Insulants are sorted on their µ value: 1 for mineral wool, 60 for EPS, 150 for XPS.
- 120 mm of XPS reaches Sd = 18 m, a vapour barrier, fitted against cold masonry.
- Bio-based products have no default value: permeability is read on the product certificate.
- A damp failure following the work falls on you, and only records dated before installation protect you.
